New filtering method promises safer drinking water, improved industrial production

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New filtering method promises safer drinking water, improved industrial production

New filtering method promises safer drinking water, improved industrial production
Could curb a drinking water-related disease that affects tens of millions of people

U.S. National Science Foundation-funded scientists at Tufts University have developed a new filtering technology inspired by biology that could help curb a drinking water-related disease that affects tens of millions of people worldwide. It could also potentially improve environmental remediation and industrial and chemical production, among other processes.

Reporting in Proceedings of the National Academy of Sciences, the researchers demonstrated that their novel polymer membranes can separate fluoride from chloride and other ions -- electrically charged atoms -- with twice the selectivity reported by other methods. The scientists say application of the technology could prevent fluoride toxicity in water supplies where the element occurs naturally at levels too high for human consumption.

The research was funded by America's Seed Fund powered by NSF, also known as the Small Business Innovation Research (SBIR)/Small Business Technology Transfer (STTR) program.

"America's Seed Fund supports companies' technical work on the most promising advances and solutions to high-impact problems," said Anna Brady-Estevez, a program director in NSF's Directorate for Engineering. "Fluoride poisoning causes severe health issues globally, and technologies such as these offer the opportunity to reduce fluoride in water to healthy levels."

It is well known that adding fluoride to a water supply can reduce the incidence of tooth decay, including cavities. Less well known is the fact that some groundwater supplies have such high natural levels of fluoride that they can lead to severe health problems. 

Prolonged exposure to excess fluoride can cause fluorosis, a condition that can weaken the teeth, and calcify tendons and ligaments. The World Health Organization estimates that excessive fluoride concentrations in drinking water have caused tens of millions of dental and skeletal fluorosis cases worldwide.

The ability to remove fluoride with a relatively inexpensive filtering membrane could protect communities from fluorosis without requiring the use of high-pressure filtration or having to completely remove all components and then remineralize the drinking water.

"The potential for ion-selective membranes to reduce excess fluoride in drinking water supplies is very encouraging," said Tufts scientist Ayse Asatekin. "But the technology's potential usefulness extends beyond drinking water to other challenges. The method we used to manufacture the membranes is easy to scale up for industrial applications." 

 

Significance

The separation of ions is challenging yet crucial for providing access to safe water resources as well as recovering valuable ions from water and wastewater. Yet, membranes rarely exhibit selectivity between ions of similar charge and size. We demonstrate that membranes, prepared by a fully scalable method that uses self-assembling zwitterionic copolymers, exhibit exceptional selectivity between salt anions of similar size and charge. We show that this unusual capability is derived from selective zwitterion–ion interactions occurring within the nanochannels, similarly to biological ion channels. We further demonstrate these membranes exhibit Cl−/F− permselectivity more than twice the values reported in previous studies, with applications in treating groundwater streams to prevent fluorosis and in wastewater treatment.

Abstract

Water filtration membranes with advanced ion selectivity are urgently needed for resource recovery and the production of clean drinking water. This work investigates the separation capabilities of cross-linked zwitterionic copolymer membranes, a self-assembled membrane system featuring subnanometer zwitterionic nanochannels. We demonstrate that selective zwitterion–anion interactions simultaneously control salt partitioning and diffusivity, with the permeabilities of NaClO4, NaI, NaBr, NaCl, NaF, and Na2SO4 spanning roughly three orders of magnitude over a wide range of feed concentrations. We model salt flux using a one-dimensional transport model based on the Maxwell–Stefan equations and show that diffusion is the dominant mode of transport for 1:1 sodium salts. Differences in zwitterion–Cl− and zwitterion–F− interactions granted these membranes with the ultrahigh Cl−/F− permselectivity ( P Cl- /P F-  = 24), enabling high fluoride retention and high chloride passage even from saline mixtures of NaCl and NaF.

Membranes with advanced ion selectivity could provide a sustainable technical solution to global resource shortages. For example, dangerously high concentrations of fluoride in available drinking water sources affects many communities, resulting in widespread, debilitating illnesses such as fluorosis (1, 2). Membranes with enhanced chloride/fluoride selectivity could protect these communities from fluorosis without necessitating high-pressure filtration or remineralization of the drinking water (2, 3). Limited geological reserves of lithium and uranium pose a major challenge to sustainable lithium battery production and nuclear power generation, respectively (4). Membranes capable of selective ion retention could enrich aqueous feedstocks with desired ions, enabling efficient capture of precious metals (5). Current synthetic membranes separate solutes by size and charge differences (6, 7), limiting their use in advanced ion separations such as these. As such, the design of synthetic membrane filters with targeted ion selectivity represents a crucial challenge for addressing global resource shortages.

Biological ion channels (BICs) exhibit exquisite ion selectivity that can inspire novel membranes capable of precise ion separations. Potassium channels permeate potassium >10,000 times faster than sodium, despite the sub-Å size difference between these equally charged ions (8). While synthetic membranes separate ions primarily by size and charge differences (6, 7), BICs rely on interactive functional groups that line the walls of nanopores (6, 9). The favorability and strength of these interactions control ion partitioning and diffusion rate, providing a powerful mechanism for separating similarly sized ions (6, 9). Crucially, BIC pore diameters are comparable to or slightly smaller than the hydrated diameter of the target ion (6, 9). This nanoconfinement forces ions to interact with the functional groups lining the pores, greatly amplifying selectivity

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SOURCES https://www.pnas.org/content/118/37/e2022198118, https://www.nsf.gov/discoveries/disc_summ.jsp?cntn_id=303584

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